From a2639824e8458a8f3826e184081a13bfa27ea884 Mon Sep 17 00:00:00 2001 From: Jan Beulich Date: Fri, 19 Feb 2021 17:21:12 +0100 Subject: [PATCH] x86/time: adjust time recording in time_calibration_tsc_rendezvous() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit The (stime,tsc) tuple is the basis for extrapolation by get_s_time(). Therefore the two better get taken as close to one another as possible. This means two things: First, reading platform time is too early when done on the first iteration. The closest we can get is on the last iteration, immediately before telling other CPUs to write their TSCs (and then also writing CPU0's). While at the first glance it may seem not overly relevant when exactly platform time is read (when assuming that only stime is ever relevant anywhere, and hence the association with the precise TSC values is of lower interest), both CPU frequency changes and the effects of SMT make it unpredictable (between individual rendezvous instances) how long the loop iterations will take. This will in turn lead to higher an error than neccesary in how close to linear stime movement we can get. Second, re-reading the TSC for local recording is increasing the overall error as well, when we already know a more precise value - the one just written. Signed-off-by: Jan Beulich Acked-by: Roger Pau Monné Release-Acked-by: Ian Jackson --- xen/arch/x86/time.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c index 23971ed7ac..a894f74b88 100644 --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -1662,11 +1662,12 @@ struct calibration_rendezvous { }; static void -time_calibration_rendezvous_tail(const struct calibration_rendezvous *r) +time_calibration_rendezvous_tail(const struct calibration_rendezvous *r, + uint64_t tsc) { struct cpu_time_stamp *c = &this_cpu(cpu_calibration); - c->local_tsc = rdtsc_ordered(); + c->local_tsc = tsc; c->local_stime = get_s_time_fixed(c->local_tsc); c->master_stime = r->master_stime; @@ -1691,11 +1692,11 @@ static void time_calibration_tsc_rendezvous(void *_r) while ( atomic_read(&r->semaphore) != (total_cpus - 1) ) cpu_relax(); - if ( r->master_stime == 0 ) - { - r->master_stime = read_platform_stime(NULL); + if ( r->master_tsc_stamp == 0 ) r->master_tsc_stamp = rdtsc_ordered(); - } + else if ( i == 0 ) + r->master_stime = read_platform_stime(NULL); + atomic_inc(&r->semaphore); if ( i == 0 ) @@ -1720,7 +1721,7 @@ static void time_calibration_tsc_rendezvous(void *_r) } } - time_calibration_rendezvous_tail(r); + time_calibration_rendezvous_tail(r, r->master_tsc_stamp); } /* Ordinary rendezvous function which does not modify TSC values. */ @@ -1745,7 +1746,7 @@ static void time_calibration_std_rendezvous(void *_r) smp_rmb(); /* receive signal /then/ read r->master_stime */ } - time_calibration_rendezvous_tail(r); + time_calibration_rendezvous_tail(r, rdtsc_ordered()); } /* -- 2.39.5